Dressings and methods for treating a tissue site on a patient

ABSTRACT

Dressings, systems, and methods for treating a tissue site on a patient involve allowing liquids from the tissue site or a priming fluid to evaporate and exit the dressing through a liquid-impermeable, vapor-permeable membrane. The dressing is able to process more liquids than would otherwise be possible without evaporation and potentially to create reduced pressure. Other dressings, systems, and methods are disclosed.

RELATED APPLICATION

The present invention claims the benefit, under 35 USC §119(e), of thefiling of U.S. Provisional Patent Application Ser. No. 61/359,205,entitled “Evaporative Body Fluid Containers and Methods,” filed 28 Jun.2010, which is incorporated herein by reference for all purposes; U.S.Provisional Patent Application Ser. No. 61/325,115, entitled“Reduced-Pressure Sources, Systems, and Methods Employing A Polymeric,Porous, Hydrophobic Material,” filed 16 Apr. 2010, which is incorporatedherein by reference for all purposes; and U.S. Provisional PatentApplication Ser. No. 61/359,181, entitled “Dressings and Methods ForTreating a Tissue Site On A Patient,” filed 28 Jun. 2010, which isincorporated herein by reference for all purposes.

BACKGROUND

The present disclosure relates generally to medical treatment systemsand, more particularly, but not by way of limitation, to dressings,methods, and systems for treating a tissue site on a patient.

Clinical studies and practice have shown that providing reduced pressurein proximity to a tissue site augments and accelerates the growth of newtissue at the tissue site. The applications of this phenomenon arenumerous, but application of reduced pressure has been particularlysuccessful in treating wounds. This treatment (frequently referred to inthe medical community as “negative pressure wound therapy,” “reducedpressure therapy,” or “vacuum therapy”) provides a number of benefits,which may include faster healing and increased formulation ofgranulation tissue. It is believed that treatment of low-severity woundswith reduced pressure will yield benefits as well, but issues remain.

SUMMARY

According to an illustrative embodiment, a dressing for treating atissue site on a patient includes a wound interface member for placingproximate to the tissue site and a covering. At least a portion of thecovering comprises an evaporative window having a liquid-impermeable,vapor-permeable membrane. The covering is operable to form a sealedspace over the tissue site. The liquid-impermeable, vapor-permeablemembrane allows evaporated liquids, e.g., water vapor or a primingfluid, in the sealed space to egress the sealed space. The egress ofevaporated liquids may create a reduced pressure in the sealed space.

According to an illustrative embodiment, a method for treating a tissuesite with reduced pressure includes placing a wound interface memberproximate to the tissue site and covering the wound interface memberwith a covering having an evaporative window. The evaporative windowcomprises a liquid-impermeable, vapor-permeable membrane. The coveringforms a sealed space over the wound interface member. The method furtherincludes allowing liquids from the tissue site to evaporate and toegress the sealed space through the liquid-impermeable, vapor-permeablemembrane. The evaporation may cause a reduced pressure to be created inthe sealed space.

According to still another illustrative embodiment, a method ofmanufacturing a dressing for treating a tissue site includes providing afoam member for placing proximate to the tissue site, providing acovering formed with an evaporative-window opening, and associating aliquid-impermeable, vapor-permeable membrane with the evaporative-windowopening to form an evaporative window. The liquid-impermeable,vapor-permeable membrane is operable to allow evaporated liquids to betransmitted through the liquid-impermeable, vapor-permeable membrane.

Other features and advantages of the illustrative embodiments willbecome apparent with reference to the drawings and detailed descriptionthat follow

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, perspective view, with a portion shown in crosssection, of an illustrative embodiment of a dressing for treating atissue site;

FIG. 2 is a schematic, cross section of an illustrative embodiment of adressing for treating a tissue site wherein the dressing includes anabsorbent layer and a removable release liner;

FIG. 3 is a schematic, cross section of an illustrative embodiment of adressing for treating a tissue site wherein the dressing includes aone-way valve;

FIG. 4 is a schematic, cross section of an illustrative embodiment of adressing for treating a tissue site wherein the dressing includes apressure-regulating valve and a one-way valve;

FIG. 5 is a schematic, cross section of an illustrative embodiment of adressing for treating a tissue site wherein the dressing includes afirst foam and a second foam;

FIG. 6 is a schematic, cross section of an illustrative embodiment of adressing for treating a tissue site wherein the dressing includesbellows;

FIG. 7 is a schematic, cross section of an illustrative embodiment of adressing for treating a tissue site wherein the dressing includes acontainer with a priming fluid; and

FIG. 8 is a schematic, cross section of an illustrative embodiment of adressing for treating a tissue site.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the following detailed description of the illustrative embodiments,reference is made to the accompanying drawings that form a part hereof.These embodiments are described in sufficient detail to enable thoseskilled in the art to practice the invention, and it is understood thatother embodiments may be utilized and that logical structural,mechanical, electrical, and chemical changes may be made withoutdeparting from the spirit or scope of the invention. To avoid detail notnecessary to enable those skilled in the art to practice the embodimentsdescribed herein, the description may omit certain information known tothose skilled in the art. The following detailed description is,therefore, not to be taken in a limiting sense, and the scope of theillustrative embodiments are defined only by the appended claims.

Referring to the drawings and initially to FIG. 1, a dressing 100 fortreating a tissue site 102 on a patient 104 is presented. The dressing100 includes, inter alia, a covering 108 that allows evaporated liquidsto egress the dressing 100. The tissue site 102 may be the bodily tissueof any human, animal, or other organism, including bone tissue, adiposetissue, muscle tissue, dermal tissue, vascular tissue, connectivetissue, cartilage, tendons, ligaments, or any other tissue.

The dressing 100 includes a wound interface member, such as a foammember 106 or gauze, for placing proximate to the tissue site 102. Thedressing 100 also includes the covering 108. At least a portion of thecovering 108 is formed with an evaporative-window opening or aperture128 covered by a liquid-impermeable, vapor-permeable membrane 112 toform an evaporative window 110. The covering 108 forms a sealed space114 over the tissue site 102 and the foam member 106. Theliquid-impermeable, vapor-permeable membrane 112 is operable to allowliquids, e.g., water, in the sealed space 114 that have evaporated toegress the sealed space 114 as suggested by arrows 116. For example,body fluids from the tissue site 102 may be drawn into the foam member106 and water from the body fluids may evaporate to form water vaporthat egresses the sealed space 114 through the liquid-impermeable,vapor-permeable membrane 112.

The foam member 106 may be placed proximate to the tissue site 102 andhelps to remove liquids, e.g., body fluids, from the tissue site 102when under the influence of reduced pressure and may help distributereduced pressure to the tissue site 102. Thus, the foam member 106serves as a manifold. The term “manifold” as used herein generallyrefers to a substance or structure that is provided to assist inapplying reduced pressure to, delivering fluids to, or removing fluidsfrom the tissue site 102. The foam member 106, or manifold, typicallyincludes a plurality of flow channels or pathways that distribute fluidsprovided to and removed from the tissue site 102 around the foam member106. In one illustrative embodiment, the flow channels or pathways areinterconnected to improve distribution of fluids provided or removedfrom the tissue site 102. The foam member 106 may be a biocompatiblematerial that is capable of being placed in contact with the tissue site102 and distributing reduced pressure to the tissue site 102. Examplesof wound interface members may include, for example, without limitation,devices that have structural elements arranged to form flow channels,such as, for example, cellular foam, open-cell foam, porous tissuecollections, liquids, gels, and foams that include, or cure to include,flow channels.

More generally, the wound interface member 118 may be porous and may bemade from foam, gauze, felted mat, or any other material suited to aparticular biological application. In one embodiment, wound interfacemember 118 comprises the foam member 106 and is a porous foam thatincludes a plurality of interconnected cells or pores that act as flowchannels. The porous foam may be a polyurethane, open-cell, reticulatedfoam such as GranuFoam® material manufactured by Kinetic Concepts,Incorporated of San Antonio, Tex. In one illustrative embodiment, thefoam member 106 is a polyurethane foam—either hydrophobic orhydrophilic. The foam member 106 may be another open cell flexiblepolymer foam, such as a polyolefin, ethylene vinyl acetate, silicone,fluorosilicone, a fluoroelastomer, styrene, butadiene, or a thermoplastic elastomer. The foam member 106 may also be any of the othermaterials mentioned herein. In some situations, the foam member 106 mayalso be used to distribute fluids such as medications, antibacterials,growth factors, and various solutions to the tissue site 102. Otherlayers may be included in or on the foam member 106, such as absorptivematerials, wicking materials, hydrophobic materials, and hydrophilicmaterials.

The foam member 106 may contain or be partially or fully saturated witha priming fluid. The priming fluid evaporates from the foam member 106to help create a reduced pressure within the sealed space 114. As usedherein, “creating of reduced pressure” or like expressions includes,without limitation, augmenting reduced pressure created or supplied byother means. The priming fluids or priming solvents may be distilledwater, water treated to remove ionic content, water, or organic solventssuch as alcohols and other polar solvents. In embodiments without apriming fluid, evaporation of liquids, e.g., water, from the wound alonemay create the reduced pressure. In general, the evaporation of theliquids from the tissue site 102 or a priming fluid may cause a reducedpressure to be developed within the sealed space 114. As used throughoutthis document, “or” does not require mutual exclusivity. While theamount and nature of reduced pressure applied to a tissue site willtypically vary according to the application, the reduced pressure willtypically be between −5 mm Hg (−667 Pa) and −300 mm Hg (−39.9 kPa) andmore typically between −75 mm Hg (−9.9 kPa) and −300 mm Hg (−39.9 kPa).For example, and not by way of limitation, the pressure may be −12,−12.5, −13, −14, −14.5, −15, −15.5, −16, −16.5, −17, −17.5, −18, −18.5,−19, −19.5, −20, −20.5, −21, −21.5, −22, −22.5, −23, −23.5, −24, −24.5,−25, −25.5, −26, −26.5 kPa or another pressure.

As used herein, “reduced pressure” generally refers to a pressure lessthan the ambient pressure at a tissue site that is being subjected totreatment. In most cases, this reduced pressure will be less than theatmospheric pressure at which the patient is located. Alternatively, thereduced pressure may be less than a hydrostatic pressure at the tissuesite. Unless otherwise indicated, quantitative values of pressure statedherein are gauge pressures. Consistent with the use herein, unlessotherwise indicated, an increase in reduced pressure or vacuum pressuretypically refers to a relative reduction in absolute pressure.

The wound interface member 118, e.g., foam member 106, is covered by thecovering 108 to create the sealed space 114. The covering 108 has afirst side 120 and a second, patient-facing side 122. The second,patient-facing side 122 of the covering 108 may be placed proximate to,or over, the foam member 106. The covering 108 may include a firstportion 124 and a second portion 126. The first portion 124 may be anymaterial that provides a fluid seal. The first portion 124 may, forexample, be an impermeable or semi-permeable, elastomeric material.

“Elastomeric” means having the properties of an elastomer. Elastomersgenerally refers to a polymeric material that has rubber-likeproperties. More specifically, most elastomers have ultimate elongationsgreater than 100% and a significant amount of resilience. The resilienceof a material refers to the material's ability to recover from anelastic deformation. Examples of elastomers may include, but are notlimited to, natural rubbers, polyisoprene, styrene butadiene rubber,chloroprene rubber, polybutadiene, nitrile rubber, butyl rubber,ethylene propylene rubber, ethylene propylene diene monomer,chlorosulfonated polyethylene, polysulfide rubber, polyurethane (PU),EVA film, co-polyester, and silicones. Additional, specific examples ofthe first portion 124 include a drape with adhesive on a patient-facingside, silicone drape, 3M Tegaderm® drape, or PU drape such as oneavailable from Avery Dennison Corporation of Pasadena, Calif.

The second portion 126 of the covering 108 is the liquid-impermeable,vapor-permeable membrane 112. The liquid-impermeable, vapor-permeablemembrane 112 allows vapor to exit, or egress, as suggested by arrows116. At the same time, the liquid-impermeable, vapor-permeable material136 allows a reduced pressure to be maintained within the container 104.The liquid-impermeable, vapor-permeable membrane 112 prevents liquidtransmission. The liquid-impermeable, vapor-permeable membrane 112 isany material that is capable of preventing liquids from ingress oregress through the material and yet is operable to permit vapor, e.g.,water vapor, to egress or to be transmitted through the material.Non-limiting, illustrative examples of the liquid-impermeable,vapor-permeable membrane 112 include high moisture vapor transmissionrate (MVTR) films or other structures formed from hydrophilic polymers.Illustrative materials may include polyvinyl alcohol, polyvinyl acetate,cellulose based materials (ethers, esters, nitrates, etc.), polyvinylpyrrolidone, polyurethanes, polyamides, polyesters, polyacrylates andpolymethacrylates, or polyacrylamides. Vapor permeability as used withrespect to the liquid-impermeable, vapor-permeable material referenceswater vapor permeability.

The materials for the liquid-impermeable, vapor-permeable membrane 112may be crosslinked, blended, grafted, or copolymerized with each other.The materials for the liquid-impermeable, vapor-permeable membrane 112may be surface treated to enhance hydrophylicity. The surface treatmentsmay include chemical, plasma, light (UV), corona, or other ionizingradiation. The material for forming the liquid-impermeable,vapor-permeable membrane 112 may also be formed by forming (e.g.,casting) films and crosslinking some of the natural gums, such as guar,xanthan and alginates, or gelatin. The materials for forming theliquid-impermeable, vapor-permeable membrane 112 typically also serve asa bacteria barrier. While the material for forming theliquid-impermeable, vapor-permeable membrane 112 is fairly impervious tonitrogen and oxygen, the material is pervious to water vapor. Onespecific, non-limiting example of a suitable material is a 15 micronsheet of Hytrel APA60015 from DuPont E I De Nemours & Co., ofWilmington, Del.

According to one illustrative embodiment, an evaporative window 110 maybe formed in the covering 108. The evaporative window 110 is any portionof the covering 108 that allows vapor egress and disallows liquidegress. In one embodiment, the material from which the covering 108 isformed allows vapor transmission, but a material or coating may beapplied on or positioned adjacent to the covering 108, e.g., apatient-facing side, to inhibit the transmission of vapor through thecovering at least at certain locations. For example, an adhesive may bepositioned on the periphery of the covering 108 to mate the covering 108with the tissue surrounding the wound or tissue site 102 and be omittedat the desired location of the evaporative window 110. Theliquid-impermeable, vapor-permeable membrane 112 may be the samematerial as the covering 108 but without an adhesive or other substancethat inhibits vapor transmission.

Referring to FIG. 2, an evaporative window 110 includes anevaporative-window opening, or aperture 128 (or apertures) formed in thecovering 108. The evaporative-window opening or apertures 128 arecovered by the liquid-impermeable, vapor-permeable membrane 112. Theliquid-impermeable, vapor-permeable membrane 112 is adhered to the firstportion 124 of the covering 108 over the aperture 128 to from theevaporative window 110.

According to one illustrative embodiment, a method for treating a tissuesite 102 on a patient 104 includes placing the wound interface member118, e.g., foam member 106, proximate to the tissue site 102. The firstportion 124 of the covering 108 may be releaseably secured to thepatient's skin 130 outboard of the tissue site 102 being treated. Thecovering 108 is used to form the sealed space 114 over the tissue site102. Liquids from the tissue site 102 are drawn into the foam member 106by the hydrophilic nature of the foam member 106 and by the creation ofreduced pressure from the evaporation of a priming liquid inside of thefoam member 106. As the priming liquid evaporates, a reduced pressuremay be created in the sealed space 114. In one illustrative embodiment,evaporation of the priming liquid or body fluid creates reduced pressuresuch that no external source of reduced pressure is utilized and yetreduced pressure adequate for therapy is applied to the sealed space114. The evaporated liquids, or vapor, from the tissue site 102 or theevaporated priming liquid, exit the sealed space 114 through theliquid-impermeable, vapor-permeable membrane 112.

Because of the evaporation of liquids from the tissue site 102, thedressing 100 is able to receive and process more liquids over time thanthe dressing 100 is capable of retaining at one time. For example, inone illustrative embodiment, the dressing 100 holds 30 cc of liquid,i.e., the dressing 100 becomes completely saturated with 30 cc ofliquid, but with evaporation, may receive over time 40 cc of liquid. Inthis example, the dressing 100 processes 133 percent of capacitymeasured by saturation capacity. In other embodiments, the dressing 100may process 110%, 120%, 130%, 140%, 150%, 160%, or more of capacitybased on saturation capacity.

Referring primarily to FIG. 2, an illustrative embodiment of a dressing100 for treating a tissue site 102 is presented. The dressing 100 ofFIG. 2 is the same as the dressing 100 of FIG. 1 with two mainmodifications. First, an absorbent layer 132 has been disposed betweenthe second, patient-facing side 122 of the covering 108 and the firstside 107 of the foam member 106. The absorbent layer 132 may be anylayer of material for receiving and storing liquids, such as, an ionictype, which is based on cross linked acrylic, methacrylic, andsulphonated acrylic polymers. The absorbent layer 132 may also be anon-ionic type, which is based on water soluble polymers, such asalginates, carboxymethy cellulose, polyvinyl pyrrolidone, or otherabsorbent material. The absorbent layer 132 receives and retains fluidsfrom the tissue site 102 and enhances the capacity of the dressing 100.Other layers, including a wicking layer (not shown) may be added aswell. If a wicking layer is added, it pulls liquids and retains them ata desired location. For example, the wicking layer may pull liquids andretain the liquids against the liquid-impermeable, vapor-permeablemembrane.

The second main difference is the inclusion of a removable release liner134, or activation layer. The removable release liner 134 provides abarrier to vapor that might otherwise egress through theliquid-impermeable, vapor-permeable membrane 112. The removable releaseliner 134 allows a priming fluid to be in the foam member 106 prior touse and yet not evaporate until the removable release liner 134 isremoved.

Referring now primarily to FIG. 3, an illustrative embodiment of adressing 100 for treating a tissue site, such as tissue site 102 inFIGS. 1 and 2, is presented. The dressing 100 of FIG. 3 is analogous tothe dressing 100 of FIG. 1 except for two main differences. First, aone-way valve 136 is included. The one-way valve 136 extends through acovering 108 and into a sealed space 114. The one-way valve 136 may alsoextend through a liquid-impermeable, vapor-permeable membrane 112. Whenthe covering 108 is pressed towards the tissue site 102 (FIG. 1), theair in the sealed space 114 is evacuated through the one-way valve 136as suggested by arrow 138. The removal of air or other gasses fromwithin the sealed space 114 helps to develop a reduced pressure in thesealed space 114. The evacuation of the gases through the one-way valve136 or the evaporation and egress of liquids from the sealed spacecreates reduced pressure in the sealed space 114. A reduced pressuretreatment may, thus, occur at the tissue site without requiring aseparate reduced-pressure source.

In the embodiment of FIG. 3, the covering 108 includes a plurality ofapertures 140, or evaporative-window openings, that comprise theevaporative window 110. The apertures 140 are covered by theliquid-impermeable, vapor-permeable membrane 112 to form the evaporativewindow 110. With respect to the second difference, the embodiment ofFIG. 3 also includes an absorbent layer 132.

Referring now primarily to FIG. 4, an illustrative embodiment of adressing 100 for treating a tissue site, such as tissue site 102 inFIGS. 1 and 2, is presented. The dressing 100 of FIG. 4 is similar tothe dressing 100 in FIG. 1, but includes a one-way valve 136 and alsoincludes a pressure-regulating valve 142. The one-way valve 136 extendsthrough the covering 108 and into the sealed space 114 as in FIG. 3, andthe pressure-regulating valve 142 also extends through the covering 108and into the sealed space 114.

The pressure-regulating valve 142 allows ambient pressure or ambient airto enter into the sealed space 114 when a maximum reduced pressure valueis reached within the sealed space 114. The introduction of the ambientair lowers the reduced pressure and keeps the reduced pressure below amaximum value. The pressure-regulating valve 142 may take numerousforms, but is shown as having a ball 144 with a biasing member 146 thatbiases the ball 144 against an aperture 148. When the reduced pressurecommunicated into a chamber 150 from the sealed space 114 is high enoughto cause the ball 144 to become unseated from the aperture 148, air isintroduced into the chamber 150 and the air is communicated into thesealed space 114. Through this action, the pressure within the sealedspace 114 may be regulated to be below a maximum reduced-pressure value.

Referring now primarily to FIG. 5, an illustrative embodiment of adressing 200 is presented. The dressing 200 is analogous to dressing 100of FIG. 1 in many respects. In this embodiment, however, the dressing200 includes a foam member 206 that comprises a first foam 252 and asecond foam 254. The first foam 252 may be an annular member having afirst rigidity. The second foam 254 may be a circular insert that goesinto an annular interior portion of the first foam 252 and that has asecond rigidity. The first rigidity is greater than the second rigidity.Thus, when under reduced pressure, the second foam 254 will collapsemore readily than the first foam 252. The more rigid first foam 252 mayhelp to develop a vacuum within a sealed space 214 by serving as aninternal biasing member that forces a covering 208 away from thepatient.

The foam member 206 is covered with the covering 208 that extends beyondthe margins of the tissue site being treated. The covering 208 has afirst portion 224 and a second portion 226. The first portion 224 of thecovering 208 is typically outboard of the foam member 206. The covering208 may be secured to the patient's skin using an attachment device 256,such as an adhesive or hydrocolloid. The attachment device 256 may havea release liner covering a patient facing side 257 before deployment.The covering 208 itself may be a liquid-impermeable, vapor-permeablematerial, and at least a second portion 226 has no adhesive or othersubstances that might impair vapor transmission.

A priming liquid may be disposed within the foam member 206. In order toprevent premature evaporation of the priming liquid before the dressing200 is deployed on a tissue site, a removable release liner 234 may bereleasably attached to a first side 220 of the covering 208 over thesecond foam 254. The removable release liner 234 may be the same size orlarger than the foam member 206. The removable release liner 234 isremoved at the time of deployment on a patient. Analogous to dressing100 of FIG. 4, the dressing 200 may include a one-way valve 236 and apressure-regulating valve 242.

Still referring to FIG. 5, in operation according to one illustrativeembodiment, the foam member 206 is placed proximate to the tissue site,such as tissue site 102 of FIG. 1, that is to be treated. If not alreadydone, the cover 208 is placed over the foam member 206 and theattachment device 256 is used to secure the covering 208 to thepatient's skin. The removable release liner 234 may be removed. Thedressing 200 may be physically primed by depressing the dressing 200toward the tissue site to cause at least the second foam 254 topartially collapse forcing air through the one-way valve 236 to place aninitial reduced pressure into the sealed space 214. The reduced pressurewithin sealed space 214 is further enhanced as liquids from the tissuesite or the priming fluid evaporate and egress through the secondportion 226 of the covering 208.

Referring now primarily to FIG. 6, an illustrative embodiment of adressing 300 is presented. The dressing 300 is analogous in mostrespects to the dressing 100 of FIG. 2. The dressing 300 includes a foammember 306 for placing proximate to a tissue site, such as tissue site102 in FIG. 1, and an absorbent layer 332 for receiving and retainingliquids from the tissue site. The foam member 306 and absorbent layer at332 are covered by a covering 308. The covering 308 forms a sealed space314 over the tissue site and the foam member 306. The covering 308 has afirst portion 324 and a second portion 326. The second portion 326includes an evaporative window 310. A liquid-impermeable,vapor-permeable membrane 312 is associated with the evaporative window310 and allows the egress of evaporated liquids from within the sealedspace 314 to an exterior of the dressing 300. A one-way valve 336extends from an exterior of the dressing 300 into the sealed space 314and allows gas within the sealed space 314 to exit the one-way valve 336but disallows gas from entering.

The dressing 300 further includes a biasing member that when in positionbiases the covering 306 away from the patient. For example, the biasingmember may comprise bellows 360 having a first end 362 and a second end364. The first end 362 engages a tissue-facing side 322 of the covering306 and the second end 364 engages the patient's skin (not shown). Inoperation according to one illustrative embodiment, after deploying thedressing 300, the dressing 300 may be pressed against the patientcausing air in the sealed space 314 to exit the one-way valve 336 andsimultaneously to cause biasing member, e.g., the bellows 360, to atleast partially collapse or compress. When at least partiallycompressed, the bellows 360 seek to return to their free or naturalposition and thereby place a force on the covering 306 that pulls thecovering 308 away from the patient. This action helps to create a vacuum(or greater reduced pressure) within the sealed space 314.

Referring now primarily to FIG. 7, an illustrative embodiment of adressing 100 is presented. The dressing 100 is analogous to the dressing100 of FIG. 1 in most respects. In this embodiment, however, a container166 containing a priming fluid 168 is disposed on a first side 107 of afoam member 106. The container 166 may include a weakened portion 170that is adapted to fail before other portions of the container 166. Inother embodiments, any type of valve may be included in lieu of theweakened portion 170, wherein the valve retains the primary fluid untilthe priming fluid is needed and then allows the priming fluid out underthe influence of reduced pressure.

A covering 108 may cover the foam member 106 and form a sealed space114. The cover 108 includes an evaporative window 110 formed with anaperture 128, or evaporative-window opening. The evaporative window 110includes a liquid-impermeable, vapor-permeable membrane 112 that coversthe aperture 128 and extends beyond the aperture 128 in thisillustrative embodiment. Thus, a portion of the liquid-impermeable,vapor-permeable membrane 112 is disposed between the first portion 124of the covering 108 and the foam member 106. A removable release liner134 may cover the evaporative window 110 until time for use.

Still referring primarily to FIG. 7, according to one illustrativeembodiment, in operation, the foam member 106 is placed proximate to thetissue site to be treated and the covering 108 is placed over the foammember 106 and secured to the patient's skin around the tissue site. Thecontainer 166 may then be ruptured by depressing the container 166 oralternatively, may have already been ruptured against a hard surface orby other means immediately before placing on the tissue site. Onceruptured, the container 166 releases the priming fluid 168 onto the foammember 106. In some embodiments, the weakened portion 170 may beincluded to control the point of rupture. The removable release liner134 is removed from the evaporative window 110. The evaporative processmay then begin helping to create a reduced pressure within the sealedspace 114.

In another illustrative embodiment, a micro-pump, e.g., a piezoelectricpump, is included under (for the orientation shown in FIG. 1) thecovering 108. The micro-pump creates reduced pressure in the sealedspace 114. The exhaust from the micro-pump may egress through theliquid-impermeable, vapor-permeable material of the evaporative window110. Alternatively or in addition, the exhaust may egress through aspecific outlet or one-way valve.

Referring primarily to FIG. 8, a dressing 400 for treating a tissue site402 on a patient 404 is presented. The dressing 400 is analogous in mostrespects to dressing 100 of FIG. 1, except a forced-air device 460 hasbeen added. The dressing 400 includes, inter alia, a covering 408 thatallows evaporated liquids to egress the dressing 400. At least a portionof the covering 408 comprises a liquid-impermeable, vapor-permeablemembrane 412. For example, an evaporative-window opening or aperture 428may be formed through the covering 408, and the aperture 428 may becovered by the liquid-impermeable, vapor-permeable membrane 412 to forman evaporative window 410. The covering 408 forms a sealed space 414over the tissue site 102 and a wound interface member 418, e.g., a foammember 406. The liquid-impermeable, vapor-permeable membrane 412 isoperable to allow liquids, e.g., water, in the sealed space 414 thathave evaporated to egress the sealed space 414 as suggested by arrows416.

The forced-air device 460 may be any device that moves air across theliquid-impermeable, vapor-permeable membrane 412 to increase the rate ofwater vapor transport. The forced air may be intermittent or continuous.The forced-air device 460 may be a fan, an electrostatic device, or apiezoelectric pump 462. Flexible baffling 464 may be formed over theliquid-impermeable, vapor-permeable membrane 412 and is fluidly coupledto the piezoelectric pump 462 for receiving air from or delivering airto the piezoelectric pump 462. Drape tape 466 or another device may usedto help couple the piezoelectric pump 462 to the covering 408. Thepiezoelectric pump 462 may be positioned on a cushioning member 468. Theflexible baffles 464 may be any device that helps channel air from or tothe piezoelectric pump 462 over the liquid-impermeable, vapor-permeablemembrane 412. For example, the flexible baffling 464 may be aninflatable passageway 466 formed over the liquid-impermeable,vapor-permeable membrane 412 and held by an adhesive (not shown) againstat least a portion of the liquid-impermeable, vapor-permeable membrane412.

It should be understand that all the dressings presented herein may beformed on site or may be pre-assembled. The pre-assembled embodimentsmay be packaged within a sealed package.

Although the present invention and its advantages have been disclosed inthe context of certain illustrative embodiments, it should be understoodthat various changes, substitutions, permutations, and alterations canbe made without departing from the scope of the invention as defined bythe appended claims. As an illustrative, non-limiting example,components from one embodiment may be used with components of otherembodiments presented. As a non-limiting example, the one-way valve 136of FIG. 4 may be added to the dressing 100 of FIG. 7. As anothernon-limiting example, the forced-air device 460 of FIG. 8 may be addedto any of the other embodiments. As yet another non-limiting example,the release liner 134 may be applied to any of the embodiments herein.

It will be understood that the benefits and advantages described abovemay relate to one embodiment or may relate to several embodiments. Itwill further be understood that reference to “an” item refers to one ormore of those items.

The steps of the methods described herein may be carried out in anysuitable order, or simultaneously where appropriate.

Where appropriate, aspects of any of the examples described above may becombined with aspects of any of the other examples described to formfurther examples having comparable or different properties andaddressing the same or different problems.

It will be understood that the above description of preferredembodiments is given by way of example only and that variousmodifications may be made by those skilled in the art. The abovespecification, examples and data provide a complete description of thestructure and use of exemplary embodiments of the invention. Althoughvarious embodiments of the invention have been described above with acertain degree of particularity, or with reference to one or moreindividual embodiments, those skilled in the art could make numerousalterations to the disclosed embodiments without departing from thescope of the claims.

1. A dressing for treating a tissue site on a patient, the dressingcomprising: a wound interface member for placing proximate to the tissuesite; a covering, wherein at least a portion of the covering comprisesan evaporative window having a liquid-impermeable, vapor-permeablemembrane; wherein the covering is operable to form a sealed space overthe tissue site; and wherein the liquid-impermeable, vapor-permeablemembrane allows liquids in the sealed space that have evaporated toegress the sealed space.
 2. The dressing of claim 1, wherein thedressing is not in fluid communication with an external source ofreduced pressure.
 3. The dressing of claim 1, further comprising aone-way valve coupled to the covering that allows fluid in the sealedspace to egress when the covering is moved toward the tissue site. 4.The dressing of claim 1, further comprising a pressure-regulating valvecoupled to the covering for allowing ambient air into the sealed spaceto keep the pressure in the sealed space from exceeding a maximum value.5. (canceled)
 6. The dressing of claim 1, further comprising anabsorbent layer for receiving and storing liquids from the tissue site,the absorbent layer disposed between the wound interface member and thecovering.
 7. The dressing of claim 1, further comprising a removablerelease layer that is releasably attached at least over the evaporativewindow, wherein the removable release layer comprises a fluid barrier.8. (canceled)
 9. The dressing of claim 1, further comprising a containerstoring priming fluid.
 10. (canceled)
 11. The dressing of claim 6,further comprising a priming fluid in the absorbent layer.
 12. Thedressing of claim 1, wherein the evaporative window comprises aplurality of apertures in the covering that are covered by theliquid-impermeable, vapor-permeable membrane.
 13. The dressing of claim1, wherein the wound interface member comprises a foam member. 14.(canceled)
 15. The dressing of claim 1, wherein the wound interfacecomprises a foam member and further comprising a biasing member disposedaround the foam member.
 16. The dressing of claim 15, wherein thebiasing member comprises bellows around the foam member.
 17. Thedressing of claim 1, wherein the evaporative window is defined by anarea of the covering that is free of adhesive.
 18. The dressing of claim1, wherein the covering comprises a drape material having an adhesive ona portion thereof and the liquid-impermeable, vapor-permeable membranecomprises the area of the drape material substantially free of adhesive.19. The dressing of claim 1, further comprising a forced-air deviceassociated with the covering that forces air across theliquid-impermeable, vapor-permeable membrane.
 20. The dressing of claim19, wherein the forced-air device comprises a piezoelectric pump andfurther comprising flexible baffling adapted to direct air to or fromthe piezoelectric pump across the liquid-impermeable, vapor-permeablemembrane.
 21. A method for treating a tissue site with a reducedpressure, the method comprising: deploying a foam member proximate tothe tissue site; covering the foam member with a covering having anevaporative window, wherein the evaporative window comprises aliquid-impermeable, vapor-permeable membrane, the covering forming asealed space over the foam member; and allowing liquids from the tissuesite to evaporate and to egress the sealed space through theliquid-impermeable, vapor-permeable membrane.
 22. The method of claim21, wherein the step of allowing liquids from the tissue site toevaporate and to egress further comprises creating a reduced pressure inthe sealed space by the evaporation.
 23. The method of claim 21, whereinthe covering comprises a drape material having an adhesive and theliquid-impermeable, vapor-permeable membrane comprises the drapematerial without adhesive.
 24. The method of claim 21, wherein noexternal reduced pressure is applied to the sealed space and wherein areduced pressure is created in the sealed space.
 25. The method of claim21, wherein: the step of deploying the foam comprises deploying a foamcontaining a priming fluid; and the step of allowing the liquids fromthe tissue site to evaporate comprises allowing the liquids from thetissue site and priming fluid to evaporate.
 26. The method of claim 21,wherein a one-way valve is associated with the covering and furthercomprising the step the depressing the foam member to remove air throughthe one-way valve from the sealed space.
 27. The method of claim 21,further comprising fluidly coupling a pressure-regulating valve to thecovering and wherein the pressure-regulating valve is operable to openat a maximum value of reduced pressure.
 28. The method of claim 21,wherein the liquid-impermeable, vapor-permeable membrane is covered by aremovable release liner and further comprising the step of removing theremovable release liner.
 29. The method of claim 21, further comprising:disposing a container storing a priming fluid between the covering andthe foam member and pressing on the container to rupture the containerto allow the priming fluid to be released in the sealed space.
 30. Themethod of claim 21, further comprising forcing air across theliquid-impermeable, vapor-permeable membrane with a forced-air device.31.-37. (canceled)